An apparatus for sorting products provided in a continuous stream is known in the art. Such sorting apparatus comprises a transport system, an inspection system and a removal system. The transport system conveys the product stream to be inspected towards the inspection system and the removal system. The inspection system will analyze one or more predetermined characteristics of the products. Typically optical characteristics such as color and structure are being examined. Based upon the optical signals it receives, the inspection system will evaluate if the measured values of these characteristics for a given object in the product stream meet predetermined acceptance criteria. If not, this object is subsequently removed from the product stream by the removal system. Hereto the inspection system controls the operation of the removal system.
The configuration of such a sorting apparatus is disclosed by U.S. Pat. No. 6,509,537. This sorting apparatus comprises a conveyor for transporting a stream of solid particles and a device for detecting and differentiating between the quality and/or the color of the individual solid particles. The detection system comprises a laser beam, which is redirected towards the solid particles via a polygon wheel. Due to the rotation of the polygonal wheel the mirroring end surfaces of the wheel will azimuthally guide the laser beam in a temporal saw-tooth movement. The moving laser beam is then directed towards the stream of solid particles to provide a linear laser beam scan thereof. The laser beam, which is re-emitted by the solid particles in a divergent way, is redirected via the mirroring end surfaces of the wheel towards photoelectrical devices converting the optical signal into an electrical output signal. The polygon wheel has thus two functions: creating a scanning laser beam over the product stream and redirecting light returning from the product stream to photoelectrical devices. This output signal can then be further handled by analog electrical circuitry or converted into a digital signal for digital processing and data manipulation. Likewise U.S. Pat. No. 4,723,659, U.S. Pat. No. 4,634,881 and European patent application EP 1 332 353 disclose sorting devices comprising inspection systems in which the polygonal wheel has two functions as described above. Such systems include all drawbacks of the prior art due to the fact that scanning means have also the responsibility of re-directing the returning light through detection means, e.g. photomultiplier detectors, thus the systems are strictly bounded with a second function which in turn does not allow to optimize the system in particular for high speed sorting operations.
TR 2006/05534U discloses a similar sorting device configuration as above with the exception that at least one of the detectors is provided with a diaphragm (delimiting device) having a slit like aperture solely for the purpose of tolerating deviations of the incoming light, which deviation is generally caused by improper movement of the scanning means i.e. rotatably arranged polygonal mirror. The present invention provides also, as a side technical effect, a solution for these problems associated with said scanning means simply because inspection systems of the current invention receive reemitted light directly from the product stream instead of receiving the incoming light from such scanning means (e.g. polygonal mirror).
U.S. Pat. No. 6,671,042 B1 discloses an inspection system including a multiple beam laser scanning unit and at least one multiple beam dark field imaging unit. Dark field inspection system is defined as a detector collecting scattered light at an oblique angel β which is outside of the convergence angle of the post-scan optical system. The scanning unit generates multiple laser spots and scans them along a surface. The imaging unit separately detects light scattered from each of these multiple spots. Each imaging unit includes collection optics and a photodetector per spot, such that each detector detects the scattered light from only its associated spot. However, the spatial filtering means of the inspection system is located in the focal plane which limits the range of angles of all light directed towards the multiple photodetectors. This is particularly disadvantageous in detecting and inspecting irregular objects in a stream of products because it does not allow to determine from which particular area the light originated.
US-A1-2005/052644 discloses a surface inspection system comprising at least one oblique illumination beam and possibly a second illumination beam in sequence or simultaneously. The filtering means described in the specification filters in the spatial frequency domain requiring the detecting means to be in a very specific constellation as is depicted in FIG. 8. It is clear that the filter in the system is not a filter in the spatial domain, but a filter in the frequency domain of the image. It is therefore that said filter is ideally suited to filter out regular, reoccurring patterns in the image. It is further evident that the Fourier filters employed in the system, also have stringent requirements on the exact locations of the collection lenses in the detection means. That is to say, the object has to be located in the front focal plane of the first collection lens. This system is again not suitable for detecting or inspecting irregular stream of products because it generates an image in the frequency domain as opposed to the spatial domain which is a much more convenient a practical domain in which to define certain window or characteristic functions as used in the present invention and described later in this application.
EP-A-1 724 030 discloses a detection system for inspecting a continuous stream of products comprising a reference element and an intermediate optical element, means for for scanning a light beam over the product stream and means for converting the light beams re-emitted by the product stream into an electrical signal. A polygonal mirror directing the light beam towards the product stream also receives the re-emitted light and directs the same through conversion means. Therefore, scanning means has two functions, namely; directing a light beam towards product stream and receiving/directing the same upon re-emission through detectors. The image formed in said detectors are solely a spot rather than a line.
WO-A-01/07950 discloses a sorting device provided with an inspection unit, a transport system and a rejection unit wherein the inspection unit is provided with at least two light sources and means for having the electromagnetic radiation meet the products. These means function as an alignment system for the radiation originating from said sources. This alignment system simply receives the electromagnetic radiation reflected and/or transmitted and/or emitted and/or transformed by the products to be sorted. However, the system does not allow discrimination of the light scattered and directly re-emitted from the product stream.
To measure scattering effects it is essential in the prior art that the incident light is concentrated in two dimensions, more particularly concentrated as a spot of light. In that case, the image as seen by the detector is made up of two parts, namely a typically bright center spot, usually referred to as the direct reflected light, and surrounding that a cloud having an intensity dependent on the scattering properties of the illuminated object. Filtering out one of those two spatial image components is done by a two dimensional spatial window, for instance a diaphragm with a circular opening having a blinding spot. Further teaching of such diaphragms can be found in the U.S. Pat. No. 4,723,659.
The amount of light received by the photoelectrical devices is determined by the area of the mirroring end surfaces of the polygon wheel which collect light returning from the product stream. If more light is to be received by the inspection system one can either increase the power of the laser beam or increase the dimensions of the mirroring end surfaces resulting in a larger polygon wheel. Both solutions however have a negative impact on the overall cost of the apparatus. Therefore, the prior art inspection systems still need to be improved so as to obtain an inspection system that does no longer require optimization of the scanning elements in accordance with the amount of received light and that does allow implementation of the scanning elements in considerably smaller dimensions.
If the speed at which the products move through the plane of the scanning beam increases, the scanning frequency must increase proportionally in order to have all passing objects scanned with sufficient vertical resolution. In the prior art inspection systems this can be solved by increasing the rotation speed of the polygon wheel or increasing the number of mirroring end surfaces of the polygon wheel. Apart from the cost implications the latter solution will additionally impact the amount of collected light. To overcome that one should increase the polygon wheel even further, which results in even higher costs.
In some applications it may be advantageous to measure the transmittance of an object instead of its reflectance. The prior art systems which collect the returning light by the polygon wheel can only be used in a reflective mode. Light source and detectors are situated at the same side of the product stream and are optically coupled to the product stream by common optical elements (e.g. the polygon wheel) such that a static, de-scanned image of the scanned object is obtained at the detectors. One could position a second polygon wheel and corresponding detectors at the opposite side of the product stream for measuring the transmitted light. However, appropriate alignment of the frequency and phase of the polygon wheel providing the scanning beam with the polygon wheel collecting the transmitted light is extremely difficult to achieve.
Finally, the optical elements of the inspection system are critical and have a large impact on the overall cost of a sorting machine. Hence it needs to be designed with maximum reliability while keeping minimal cost in mind. These design criteria are generally met by keeping optical distances as short as possible, as few as possible degrees of freedom and stable components to obtain a high enough level of optical stability. In the above mentioned prior art systems, however, these goals can be met only to an unsatisfactory degree. To begin with, the reemitted light has to travel over a considerable optical distance and has to pass through a considerable number of optical elements like mirrors and lenses, before finally reaching the detecting means. Furthermore, these optical elements are mounted in mechanical holders fixed on a stabilized base plate. The holder itself can be pitched and yawed to align the laser beam and sometimes moved back and forth to cope, for instance, with chromatic aberrations. It is a complex task to align such optical systems as can be appreciated by a person skilled in the art. Lastly, as outlined above, the prior art systems need to increase the area of the polygonal mirroring end surfaces as much as possible. However, this has a proportional effect on the area of all other optical elements in the optical path towards the detectors, which is in contradiction with the overall design goal.
One could at least theoretically envisage other ways to de-scan and capture light using complex optical setups such as for instance parabolic mirrors. However these solutions would at least partially suffer from the same shortcoming as given above.
Hence there is a need for an apparatus for inspection of products that doesn't suffer from the shortcomings of the prior art. Thus there is a need for a sorting apparatus comprising an inspection system which doesn't suffer from the shortcomings of the prior art. Advantages of the invention will be further disclosed in the rest of the description with reference to the appended drawings as well as to the above drawbacks of the prior art in more detail.